1. Field of the Invention
This invention relates to circuits providing position information. In particular, this invention relates to linear and circular potentiometers providing a signal corresponding to position information.
2. Description of the Related Art
As seen in FIG. 1, a typical circular potentiometer consists of an element 10, tap 11, tap 12 and a wiper 13. Element 10 is a substance of known electrical conductivity, usually of much greater length than width, which is coupled to an electrically insulating substrate 14. Tap 11 and tap 12 are positioned at each end of element 10. Wiper 13 has one end coupled to a rotational center defined by the circular form of element 10. Wiper 13 is a rotatable electrically conducting member allowed to have wiper contact 18 move from, at least, tap 11 to tap 12. As wiper 13 rotates, wiper contact 18 is electrically connected to element 10.
A potentiometer is often used for providing position information by using a voltage source. A fixed voltage source is applied between tap 11 and tap 12 and then a voltage is obtained between either tap 11 or tap 12 and wiper contact 18. As wiper contact 18 is moved along element 10, the voltage between either tap 11 or tap 12 and wiper contact 18 changes as a function of the radial distance between wiper contact 18 and either tap 11 or tap 12. If element 10 is of a uniform resistance per unit length, then the voltage is a linear function of wiper contact 18 position on element 10.
Because of this convenient linear function, potentiometers may be used as transducers from position to voltage, whenever it is necessary for a system to measure the position of a mechanical device. Frequently, potentiometers are used in meteorological instrumentation systems and, in particular, used in calculating wind direction.
Typically, tap 11 and tap 12 form a small gap or open wiper angle 15. As wiper contact 18 rotates through 360 degrees, it will encounter open wiper angle 15 in which it makes no electrical contact to element 10. When wiper contact 18 is within open wiper angle 15, the potentiometer is considered in an open wiper state.
Some potentiometers incorporate stops, as shown as stop 16 and stop 17 in FIG. 1, which prevent rotation of wiper contact 18 through open wiper angle 15. Other potentiometers, in particular circular potentiometers, because of the mechanical devices to be connected to them, must be able to have wiper contact 18 rotate through open wiper angle 15. For example, wind direction mechanical devices require a freely rotating circular potentiometer which permits a wiper contact 18 rotation through open wiper angle 15.
A circuit for measuring the voltage between wiper contact 18 and either tap 11 or tap 12 (and thus the position) of wiper contact 18 is shown schematically in FIG. 2. Voltage source 20 is coupled to tap 11 and tap 12. Wiper 13 is coupled to voltage sampling measurement device 21 and resistor 22.
When a freely rotating circular potentiometer is used with the circuit shown in FIG. 2, wiper 13 will be in an open wiper angle 15 at least some of the time. Without resistor 22, voltage sampling measurement device 21 will respond to an open wiper state by measuring a voltage which drifts in either a positive or negative direction from the voltage measured just before wiper contact 18 departed from tap 11 or tap 12. The drift may be made definitively unidirectional by connecting a pulling resistor or resistor 22 between wiper 13 and node 28 whose voltage lies outside of the voltage range encompassed by the element, due to the additional voltage source 26.
However, with or without resistor 22, definitive information about the position of wiper 13, and in particular wiper contact 18 position is not available during several types of open or just-closed wiper states. Without using resistor 22, the voltage may drift in any direction; thus, a seemingly normal connected wiper voltage may be erroneously measured while wiper contact 18 is in open wiper angle 15. When resistor 22 is used, the situation is somewhat improved although still not definitive, as the position of wiper contact 18 can only be positively identified in only three out of five possible open or just-closed wiper states.
When using resistor 22, there are five possible open or just-closed states, which can be classified into two types: static states and dynamic states. The one static state is when wiper 13 is indefinitely in an open state. The position of wiper 13 in this state is readily identifiable as being within the open wiper angle. Once the voltage of wiper 13 has stabilized in the open state, the voltage is very clearly seen as being the voltage between tap 12 and node 28, which is a voltage outside of the normal "connected wiper range" (hereinafter called "CWR"), assuming a negligible current flowing in resistor 22. Such a reading is only possible when the wiper is within the open wiper angle.
The four possible dynamic states arise from the fact that voltages and voltage sampling measurement device readings do not change instantaneously. Thus, as the voltage at wiper contact 18 is sampled at various times, measurements may be obtained during brief dynamic states occurring between relatively more stable connected wiper and open wiper states. FIG. 3 shows a typical curve of measured voltage as a function of time for a wiper contact 18 rotation at constant speed, once in each direction through the open wiper angle, using the circuit of FIG. 2.
Segment 31: Wiper contact 18 has departed from element 10 at tap 12. Identifiable: (-) slope; voltage out of CWR PA1 Segment 32: Wiper contact 18 has just reached tap 11. Identifiable: high (+) slope. PA1 Segment 33: (the closed wiper state) Wiper contact 18 traces voltages within the CWR on element 10. Direction is reversed while in the CWR. Identifiable: low slope; voltage in CWR. PA1 Segment 34: Wiper contact 18 has departed from element at tap 11. Indeterminate: High and changing (-) slope as in segment 35. PA1 Segment 35: Wiper contact 18 has just reached tap 12. Indeterminate: High and changing (-) slope as in segment 34.
The wiper contact position in segment 34 and segment 35 is difficult to determine because both segments have similar high negative slopes, one of which indicates an open wiper (voltage moving toward node 28); the other of which indicates a wiper just having reached tap 12 (voltage moving toward tap 12). The high negative slopes of segments 34 and 35 could be made distinguishable and thus identifiable, but this would impose constraints on voltage measurement devices, circuitry and cabling. Voltage sampling measurement device 21 would be required to have a greater sampling rate in order to distinguish between the two high and changing negative slopes in segment 34 and segment 35. However, such high rate voltage sampling measurement devices would impose greater system costs. Likewise, length constraints would be imposed on connecting cable between the potentiometer and the voltage source and the voltage sampling measurement device because inherent cable capacitance would affect the slopes in segment 34 and segment 35, and thus the identification of wiper contact 18 position.
Therefore, it is desirable to have a circuit which provides position information of a potentiometer,wiper during all open and just-closed states without requiring expensive and complex circuitry, cabling and measurement devices.